Characterization of titanosilicate ETS-4 crystals grown from synthesis mixtures of different alkalinity

The results of physicochemical characterization of the titanosilicate ETS-4 crystallized at 448 K from synthesis mixtures of alkalinity varied in the 11.50-12.90 initial pH range [Yilmaz et al., Micropor. Mesopor. Mater., this issue] are presented. The average geometric diameter of spherulites obtained decreased whereas the size of individual crystallites forming these spherulites increased upon the increase of the alkalinity of synthesis mixtures in the range 11.50-12.80. The high initial pH of 12.90 was necessary to grow monolithic ETS-4 crystals with average dimensions ∼1 × 15 × 2 μm and characterized by rectangular prism morphology and minimal intergrowth. The Si/Ti ratio decreased from value of ∼3 upon increasing the initial pH of synthesis mixture and approached an ideal value of 2.4 at the highest alkalinity investigated (12.9). This indicated the presence of significant number of framework defects in the form of missing Ti atoms in ETS-4 grown at lower alkalinities. A layered microstructure in the form of spiral hillocks with step height of ∼1.1 nm was observed on the {1 0 0} surfaces of monolithic ETS-4 crystals. This step height nicely coincides with the geometric dimension of two building units that form the ETS-4 framework: a titanosilicate chain containing octahedral titanium, and a bridging unit containing semi-octahedral titanium. The XRD analysis confirmed the crystallite size increase in the spherulitic products obtained from synthesis mixtures of increasing alkalinity, and was consistent with the EDX analysis, which suggested a smaller number of Ti vacancies in products grown from more alkaline synthesis mixtures.